um......It has nothing to do with the fact that it is "rear-wheel drive". My 98 is a bit loose after 6 years of driving....and it's fine
If it it a safety concern, I would take it to a mustang shop and get some of the Steering parts replaced/tightened.
If the car is a bit lowered, It may need a "Bumpsteerkit" or "Camber casters".
I would have it looked at if iti s a "concern"...
